Emma Anholt authored Mar 26, 2019 and Adam Jackson committed Apr 17, 2019

We had various helper functions trying to come up with the
internalformat/format/type/render formats for pixmaps, and it's much
nicer to just detect what those should be once at startup.  This gives
us a chance to do the right thing for GLES.

It also, notably, fixes our format/type for depth 15 and 16 setup for
desktop GL, so that we actually allocate 16bpp (GL_RGB/565) on most
drivers instead of 32bpp (GL_RGB/UBYTE).

GLES still has regressions over desktop (2 regressions in llvmpipe
XTS, many in rendercheck), but I think this is a good baseline.

Signed-off-by: Eric Anholt <eric@anholt.net>